1. Field of the Invention
The invention relates to hydrophones for the reception of acoustic waves in water in shallow or deep applications. The hydrophone herein disclosed has enhanced compliance, which together with hydrostatic pressure balancing permits sensitive and linear operation down to substantial depths.
2. Description of the Prior Art
Hydrophones may be characterized as falling into two classes: compliant hydrophones, which sense acoustic wave displacements; and piezoelectric hydrophones, which sense acoustic pressures. The sensing plate in a piezoelectric hydrophone, customarily a piezoelectric ceramic disk, generates charge and voltage when compressed or bent by incoming acoustic pressure waves. The sensing plate is relatively stiff in itself, but for use at a depth, a pressure balance is normally provided. Such a system is described by Edward T. O'Neill in an article entitled "Pressure-Balanced High-Pressure Hydrophone", Journal of the Acoustic Society of America, 34(11) Oct. 1962, pages 1661-1662.
In this known system substantially equal hydrostatic pressures are maintained between the external and internal surfaces of the sensing member. The external water pressure, acting inwards, is balanced by internal oil pressure acting outwards. The balance pressure is transmitted through a flexible rubber diaphragm, through an oil reservoir, into a capillary tube and finally through the tube to the oil-filled "backing" chamber that adjoins the backside of the sensing member. Slow changes in pressure are equalized by oil flow through the small opening of the capillary tube. Its size-limited flow-rate sharply limits the speed with which pressure in the backing chamber can follow external water pressure, so high frequency pressure changes attributable to acoustic waves are essentially isolated from the pressure in the backing chamber. The design permits one to set a low frequency "cut off" at a desired value, 100 Herz being typical. To an acoustic signal impinging on the sensing member, the backing chamber is a load, in the form of an oilfilled compliant volume, where the compliance is primarily of the oil in a rigidly bounded space.
Since the compliance of an oil-filled volume is small, being formed of only slightly compressible oil, within a volume acoustically held constant, this backing chamber has a low compliance to incoming acoustic signals. With a piezoelectric sensing plate, which is relatively stiff in itself, this backing has a small effect on the internal plate deformation. Thus in the interests of high performance, one may incorporate compliance enhancing means in the interior of the transducer.
When the hydrophone is of a high compliance, both the issues of a hydrostatic pressure balance and compliance enhancement become critical to a practical device. It is essential that there be a hydrostatic pressure balance to avoid rupture, if operation at substantial ambient pressures is contemplated. The application of a low compliance oil backing to a high compliance hydrophone however, very seriously degrades the compliance and thereby the sensitivity of the hydrophone. Assuming the conventional double-chamber hydrostatic pressure balancing system as described in an article by Byron W. Tietjen, .music-flat.The Optical Grating Hydrophone", J. Acoust. Soc. Am., 69(4) Apr. 1981, page 993, calculation predicts a 100 fold decrease in hydrophone compliance with the admission of oil into the backing chamber and a corresponding decrease in hydrophone sensitivity.
It is known that one can increase hydrophone compliance and thereby the sensitivity (at atmospheric pressure) in a system using an oil-filled backing chamber by introducing a small amount of gas into the backing chamber. The gas is a high compliance material at atmospheric pressure. When present with the low compliance oil in the backing chamber, the gas (at atmospheric pressure) increases the compliance presented to the internal face of the sensing plate, and thereby the compliance of the hydrophone. Materials which add compliance when installed in a liquid filled hydrophone have been characterized as "pressure release" materials.
Gas, however, is unsatisfactory as a compliance increasing agent for most hydrophone applications. Since the pressure volume product is constant for a gas, when subject to a high hydrostatic pressure, the volume decreases in inverse proportion to pressure and the compliance as a square of that ratio. At 1500 psi, for example, the compliance of the enclosed gas is reduced from its atmospheric value by a factor of 10,000. Therefore, the effective compliance with gas inclusions reduces rapidly with pressure, producing a hydrophone sensitivity which also drops rapidly with pressure. The approach is usually unsatisfactory even at shallow depths.
It is also known that compliant structures such as slightly flattened tubes may be disposed in the water for purposes of causing acoustic refraction and scattering. Such use of compliant tubes is discussed in two articles entitled "Acoustic Refraction and Scattering with Compliant Elements" (Part I, Measurements in Water, J. Acoust. Soc. Am., Volume 29, No. 9, Sept. 1957, pages 1021-1026, and Part II, Analysis, pages 1027-1033).
In an article by Gerald A. Brigham entitled "On the Theory of Stress Constrained Optimum Compliant Tubes and Uniform Tube Arrays" J. Acoust. Soc. Am. 69(6) June 1981, pages 1545-1556 compliant tubes are again dealt with. The article deals primarily with the low frequency model for the plane wave transmissivity of a planar uniform grating of compliant tubes. The article, however, makes reference to the use of "uncompensated compliant tubes either as reflectors or as pressure release components for the interior of underwater transducers. . . . " The article implies that such useage of compliant tubes in underwater transducers is of limited usefulness but adds no details.
In a Government report entitled "Handbook of Hydrophone Element Design Technology" prepared for the Naval Electronic Systems Command by Charles LeBlanc, NUSC, Technical Document 5813, Oct. 11, 1978, a review was made of "pressure release materials", which concluded that satisfactory materials for deep water hydrophone designs do not exist. The article, considering operation up to a pressure of 10K psi (70MPa), reviews a number of suggested materials including Corprene DC100, a composite of polychloroprene and cork, (R. Higgs and L. Eriksson, "Acoustic Decoupling Properties of Corprene DC-100," J. Acoust. Soc. Am., vol. 46, no. 5 (Part 2), Nov. 1969, page 1254); onionskin paper, (R. Higgs and L. Eriksson, "Acoustic Decoupling Properties of Onionskin Paper," J. Acoust. Soc. Am., vol. 46, no. 1 (Part 2), July 1969, page 211); and Sonite, an adaption for acoustic purposes of a thermal insulator called MIN-K 2000, (A. Jabin, "Physical Properties of Sonite," Report No. 3901-26, Johns Manville Research and Engineering Center, Manville, N.J., 1970.)
The NUSC Handbook adds that a further drawback of these materials less important than the limitations in their acoustic performance at high pressure (i.e. Sonite, Corprene) is that they have to be encapsulated to prevent degradation. Nevertheless, the report writer is encouraged to state "improvements can be made in pressure release materials, even though we are a long way from achieving the ideal. . . "
The NUSC Handbook concludes, "As a last comment, let it suffice to say that pressure release materials useable to great depth will not be available in the near future, and thus emphasis should be placed on the designer's ingenuity in circumventing the need for such materials in deep submergence hydrophones."